fix: omit empty image array and bogus id from post value items

Sending image:[] causes a 400 from the Postiz API — only include the
image field when there is actual media to attach. Also remove the id:""
field which was never valid.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

artifacts/postiz-mobile/app/(tabs)/compose.tsx

@@ -143,10 +143,16 @@ export default function ComposeScreen() {
         date: postNow ? new Date().toISOString() : scheduleDate.toISOString(),
         shortLink: false,
         tags: [] as string[],
-        posts: selectedChannels.map((integrationId) => ({
+        posts: selectedChannels.map((integrationId) => {
-          integration: { id: integrationId },
+          const valueItem: { content: string; image?: Array<{ id: string; path: string }> } = {
-          value: [{ content: content.trim(), id: "", image: media }],
+            content: content.trim(),
-        })),
+          };
+          if (media.length > 0) valueItem.image = media;
+          return {
+            integration: { id: integrationId },
+            value: [valueItem],
+          };
+        }),
       };
       const body = JSON.stringify(payload);